Rare-earth doped fiber amplifiers have come into prominent use in lightwave communication systems, such as for use in optical long haul terrestrial and undersea communications, distributed local area networks and in satellite communication systems, such as communication service between communication satellites in space orbit around the earth. In these systems, a major utility of the rare-earth doped fiber gain medium, such as a fiber laser or fiber amplifier, e.g., erbium-doped fiber amplifiers (EDFAs), is power boosting such as for optical transmitters, receivers or repeaters to periodically boost the transmitted signal to levels sufficient for its reliable transmission along the entire lightwave communication system. In many of these applications, the optical fiber amplifier is physically remote from the point of initial transmission or from the point of final delivery destination so that a highly reliable communication system between the point of communicative origin and final delivery destination is necessary and required due to lack of ability to easily replace or otherwise correct faulty operating components in remote regions of the system, such as, for example, in the case of remotely located optical repeaters or high power optical transmitters in outer space satellites. One such component is the radiation laser sources used to pump the fiber amplifier. Such high power sources have recently become more available in the form of semiconductor pump lasers which are sufficiently reliable to enable the increasing use of rare-earth doped fiber amplifiers.
It is conventional practice to employ 980 nm or 1480 nm semiconductor laser diodes to pump a fiber gain medium to produce amplified optical signals in the range of about 1530 nm to about 1560 nm. Most commonly a multiple QW semiconductor laser diode operating at 980 nm is used because of its low noise figure and good conversion efficiency. The pump laser wavelength of 980 nm falls within the erbium absorption band of the amplifier. In order to enhance the reliability of the semiconductor laser diode pumped fiber gain medium, the outputs of several laser diodes may be optically coupled to a fiber multimode gain medium as illustrated in U.S. Pat. No. 5,263,036, showing a double clad fiber. Thus, if one laser diode malfunctions or becomes inoperative, the remaining laser diodes can continue to function in the pumping task. Also, the use of multiple lasers permits a decrease in the power requirements of the individual laser diode, increasing reliability without compromising the gain of the amplifier as taught in U.S. Pat. No. 5,287,216. The problem arises, however, that when one, and particularly in the case of more, laser diodes fail, an increase in total power required from the remaining laser diodes is inevitable to prevent any compromise of amplifier gain or output power. One manner of solving this problem is to employ a laser array or laser bar so that multiple emitter outputs are provided from a single monolithic laser source. While such an array can provide high pumping power, if one or more laser emitters in the array malfunction or become inoperative, emitter failure affects the quality of the output of the other emitters in the array since the emitters are closely spaced to provide a single full beam output. Such laser arrays can be replaced by broad area emitter laser diodes where several such broad area lasers together can be employed to achieve high power output for the pumping application. However, such broad area lasers have not proved to be reliable over sufficiently long periods of time, particularly since they are subject to filamentation resulting failure, operate at high temperatures and have a tendency to have larger noise figures. Also, broad area lasers need adequate cooling, such as with thermoelectric coolers, adding to the required system components and adding additional concerns for long term system reliability.
The semiconductor laser diode, used as a pumping source for an optical fiber gain medium which is used itself as a pumping source, is considered the least reliable link in an optical fiber pumping system used in a lightwave communication system because these laser devices have a comparatively shorter lifetime. Fiber amplifiers have greater long term reliability compared to semiconductor laser diodes and their lifetime behavior is determinative of the overall lifetime of the lightwave communication system particularly if the system components are in a location not suited for immediate and efficient replacement, such as in the case of outer space.
What is required for enhancing the lifetime capabilities of lightwave communication systems is to increase the long term reliability of the least reliable link, i.e., improve the long term effectiveness of the entire communication system by improving the semiconductor pumping source effectiveness, reliability and longevity in these systems.
Therefore, it is a primary object of this invention to improve the long term reliability of lightwave communication systems.
It is further object of this invention to improve the long term reliability of lightwave communication systems by providing one or more levels of built-in redundancy in the pump sources for the amplifiers employed in lightwave communication systems.
It is still another object of this invention to provide plural levels of redundancy in a high power pump source for solid state optical gain medium, optical fibers being one such medium, to extend the lifetime operation of a lightwave communication while providing longer term maintenance-free operation.